The invention relates to a stacker for flat material such as banknotes, bankpapers and the like, with a stacker drum vertically bearinged on a base plate and with a belt transport system placed tangentially with respect to the stacker drum, and the stacker drum is designed for taking up the material, using a line of suction openings, at its leading edge and, on being turned, for pulling the material out of the transport system and, after moving through a certain angle of turning, stacking it against a stopper, and in which the material is acted upon by compressed air, through blowing openings, placed after the suction openings in the direction of turning, for bending the material away from the drum before being stacked.
In the case of stackers of this design, that is to say stackers in which the sheet material is taken from the transport system by a turning stacker drum, by which it is guided up against its stopper and stacked at this position, there is, more specially in the case of high stacking speeds, a shortcoming with respect to full clearing of the back face part: the sheets placed on the roller have the tendency, after the leading edge has come up against the stopper, of slipping tangentially along the outer drum or roller face, something which in the end is responsible for folding the sheet material, generally near its middle line, so that the sheet is stacked in a folded or crumpled condition. The outcome of this is that the stack produced is unorderly and in some cases further stacking may be stopped. This effect is to be seen specially frequently in the case of long sheets which as such are not of a stiff nature, as will be the case, for example, with much used banknotes.
For putting an end to this shortcoming there has been a suggestion, see for example the German Offenlegungsschrift specification No. 2,555,306, to have a blowing nozzle on the stacker drum producing an air jet tangentially in the direction of turning of the drum. As the trailing or back edge of the sheet taken up by the stacker drum is moved past the nozzle, air is to be run inbetween the transported sheet material and the drum, so that the sheet is cleared or lifted from the drum outer face by the building up of static pressure in operation of the stacker. It has, however, turned out to be the case that, because the trailing edge of the sheet is not moved clear of the drum outer face to the necessary degree, it is hardly ever possible for air to get into the space between the drum outer face and the sheet and, in fact, the only effect of the air current is the frequent folding of the sheet material along its middle line, something which, as noted beforehand, may be responsible for unorderly stacking and the stopping of the stacking work.
In the U.S. Pat. No. 2,759,543, a suggestion has been made for a stacker in the case of which, in place of an outside air blowing nozzle, placed beside the stacker drum, the blowing air goes from the inside of the drum through separate blowing nozzles,. In the direction of turning of the drum the blowing openings are placed after the suction openings used for gripping the front or leading edge of the sheet. But for the leading edge part of the sheet itself, an air cushion is formed between the sheet and the drum as the sheet material is moved round on the drum, something which makes it more readily possible for the sheets to be cleared or stretched out before coming to the clearing stopper. Because of the blowing openings, which are placed generally near and behind the suction openings, however, more specially in the case of long and much used sheets, the middle part will be lifted clear of the drum while the trailing edge is still kept sticking to the outer face of the stacker drum. A further shortcoming in the case of this stacker is the complex control system and design of the blowing and suction openings, with which trouble in operation is frequently likely. Because each opening inside the drum has its own pressure or, in the other case, vacuum chamber, it is only possible for a limited number of openings to be placed along the length of the drum, so that the taking up the sheets by the drum becomes uncertain.